In an electric power steering apparatus, the drive power of a motor is transmitted at reduced speed via a small gear provided on the output shaft of the motor and a large gear meshed with the small gear to a steering shaft provided with the large gear, and the rotating motion of the steering shaft is assisted. As the small gear and the large gear, spur gears, helical gears or the like, having power transmission efficiency higher than that of a worm gear, are used to make the electric power steering apparatus compact (Japanese Utility Model Application Laid-open No. 62-144773 (1987)). By the use of such gears having high power transmission efficiency, the output of the motor, required for assisting steering operation, can be reduced, and the motor can be made compact. In the case that spur gears or the like are used, the steering shaft is nearly parallel with the output shaft.
In recent years, for the purpose of making the electric power steering apparatus more compact, an electric power steering apparatus comprising gears based on a special theory has been proposed, in which one set of gears is used, and the large gear thereof is made as small as possible while the speed reduction ratio required for assisting steering operation is maintained and while the gear strength applicable to practical use can be obtained securely (Japanese Patent Application Laid-open No. 11-124045 (1999)). The gears based on the special theory are gears in which the curvature of the tooth profile curve is a continuous differentiable function and changes periodically in the direction of the tooth depth.
In the electric power steering apparatus comprising the gears based on the special theory, for the purpose of securely obtaining the gear strength applicable to practical use, it is necessary to accurately process the gears and to accurately maintain the center distance between the gears accommodated in the housing.
In addition, for the purpose of smoothly rotating the gears, it is necessary to provide appropriate backlash between the tooth faces. In other words, if the backlash is too large, the problem of backlash noise, that is, large meshing noise due to the collision of the tooth faces, is caused; if the backlash is too small, the problem of unsmooth gear rotation is caused. For the purpose of solving these problems, it is necessary to maintain the center distance between the gears accommodated in the housing constant.
Conventionally, these problems are solved by accommodating the steering shaft and the output shaft in the same housing, by fitting a guide plate provided with two holes spaced by a predetermined distance on the steering shaft and the output shaft, thereby maintaining the steering shaft and the output shaft in nearly parallel with each other.
More specifically, first, the steering shaft is accommodated in the housing via two roller bearings, and the motor and the output shaft are accommodated in the housing so that the steering shaft and the output shaft are in nearly parallel with each other. At this stage, the center distance between the large gear provided on the steering shaft and the small gear provided on the output shaft is not accurately constant. Next, the steering shaft is rotatably fitted into one hole provided in the guide plate, and the output shaft of the motor is rotatably fitted into the other hole. Furthermore, a C-ring is mounted on the output shaft or the steering shaft so that the guide plate is positioned at the end of the output shaft. Even if the guide plate is subjected to a force acting to move it in the axial direction, the C-ring serves as a stopper, whereby the guide plate is not moved along the output shaft or dropped from the end of the output shaft.
In the electric power steering apparatus being produced as described above, the center distance between the large gear and the small gear can be maintained accurately. In other words, the center distance between the gears can be maintained constant in the range of the dimensional errors of the two holes provided in the guide plate; therefore, the gear strength applicable to practical use can be obtained securely, and the problem of backlash noise can be solved.
However, in the conventional configuration, in the case that the electric power steering apparatus is used for a long time, the center distance between the gears cannot be maintained accurately, thereby causing problems of being unable to securely obtain sufficient gear strength and being unable to maintain appropriate backlash between the gears. In other words, in the conventional configuration, the walls of the holes provided in the guide plate are in the state of making tight contact with the output shaft and the steering shaft to accurately maintain the center distance between the gears; in the case that the electric power steering apparatus is used for a long time in this state, the walls of the holes are inevitably worn and deformed owing to the rotation of the steering shaft and the output shaft. Hence, the center distance between the large gear and the small gear cannot be maintained accurately owing to the abrasion of the walls of the holes, and the gear strength assumed in the design stage cannot be maintained. Furthermore, because of the same reason, the backlash between the gears cannot be maintained appropriately.
Moreover, because the gears based on the special theory have special tooth face profile, problems are caused, that is, the gears cannot be processed using existing general-purpose machines, and the process accuracy of the gears cannot be inspected efficiently.